Metabolomics reveal optimal grain pre-processing (milling) toward rice

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Metabolomics reveal optimal grain preprocessing (milling) toward rice koji fermentation Sunmin Lee, Da Eun Lee, Digar Singh, and Choong Hwan Lee J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b05131 • Publication Date (Web): 02 Mar 2018 Downloaded from http://pubs.acs.org on March 3, 2018

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Journal of Agricultural and Food Chemistry

Metabolomics reveal optimal grain pre-processing (milling) toward rice koji fermentation Sunmin Lee, Da Eun Lee, Digar Singh, Choong Hwan Lee* Department of Bioscience and Biotechnology, Konkuk University, Seoul 05029, Republic of Korea; [email protected]

(S.L.);

[email protected]

(D.E.L);

[email protected]

(D.S.);

[email protected] (C.H.L)

*Corresponding author Choong Hwan Lee Department of Bioscience and Biotechnology, Konkuk University, Seoul 05029, Republic of Korea Tel.: +82-2-2049-6177; Fax: +82-2-455-4291; E-mail address: [email protected]

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ACS Paragon Plus Environment


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ABSTRACT

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A time-correlated mass spectrometry (MS) based metabolic profiling was performed for

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rice koji made using the substrates with varying degrees of milling (DOM). Overall, 67

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primary and secondary metabolites were observed as significantly discriminant among

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different samples. Notably, a higher abundance of carbohydrate (sugars, sugar alcohols,

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organic acids, phenolic acids) and lipid (fatty acids, lysophospholipids) derived metabolites

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with enhanced hydrolytic enzyme activities were observed for koji made with substrate's

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DOM 5–7, at 36 h. The antioxidant secondary metabolites (flavonoids and phenolic acid)

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were relatively higher in koji with substrate's DOM 0, followed by DOM 5 > 7 > 9 and 11, at

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96 h. Hence, we conjecture that the rice substrate pre-processing between DOM 5-7 was

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potentially optimal toward koji fermentation with end-product being rich in distinctive

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organoleptic, nutritional, and functional metabolites. The study rationalizes the substrate pre-

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processing steps vital for commercial koji making.

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KEYWORDS: Rice Koji, Degree of milling, Fermentation, Mass spectrometry, Enzyme

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activity

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1. INTRODUCTION

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Fermentation, a customary food processing and preserving method, can be traced back to

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thousands of years in pre-historic era.1 Considering the modern perspectives, the fermented

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food products are globally relished as functional foods with the capacity to function as health

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aids.2 Typically, a food fermentative bioprocess is carried out using diverse microflora

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including fungi, yeast, and bacteria to improve the nutritional value and food safety with

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enhanced organoleptic properties.3,4

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The koji, being an inextricable component or starter for numerous fermented foods,

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condiments, and beverages typical to East Asia, employed largely in food fermentative

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bioprocesses.5 Koji is made through solid-state fermentation by inoculating steamed rice

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grains with molds (A. oryzae), which germinates to secrete hydrolytic enzymes triggering

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fermentation.6 Since nearly two millennia, Aspergillus strains have been used for the koji

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fermentation, employed typically for rice wines (makgeoli, sake), soy sauce (ganjang, shoyu),

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fermented soybean (doenjang, miso), soy-pepper paste (gochujang), and distilled spirits

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(shochu).7 Particularly, rice koji fermentation with Aspergillus oryzae primarily occurs

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through carbohydrate metabolism by secreted hydrolytic enzymes.8 In particular, the

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hydrolytic enzymes secreted by A. oryzae (koji mold), modulates substrate textures through

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affecting the release of metabolic gamut such as mono-sugars, amino acids, and fatty acids.9

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In the fermentation process of any product, the starting substrate and inoculum are critical

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to the quality of fermentation end-products. Previously, we correlated structural contours and

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metabolic compositions of different rice varieties with culture growth, secreted enzymes as

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well as metabolites in koji.10 Yoshizaki et al. have also reported the effects of three types of

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rice koji (yellow, white, and black) on the volatile aroma compounds using gas

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chromatography-mass spectrometry (GC-MS) based profiling.11 Functionally, the rice is a 3



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valuable and rich source of antioxidants including phenolic compounds and has a hard husk

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(hull) that protects the endosperm. Most of these compounds are distributed in different

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portions of rice grain, particularly accessible through milling process. The commercial rice-

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milling process generates products like husk and bran (pericarp) with low-value fractions.

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After the husk is removed, the remainder is known as brown rice, which includes the bran,

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embryo, and endosperm.12 Brown rice retains its bran layer and embryo bud, while the bran

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and embryo of white rice are removed during complete milling. Liu et al. have highlighted

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the effects of varying degrees of milling (DOM) on total flavonoid and phenolic

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compositions as well as the associated antioxidant activities for brown rice .13 Determining an

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appropriate DOM is vital for improving nutrient utilization and controlling potential hazards

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associated with anti-nutritional factors to avoid nutrient loss due to over-milling.14

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Metabolomics have increasingly been used to study the nutritional, functional, and

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organoleptic aspects of fermented foods in a high-throughput manner owing to the

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technological and computational advancements in mass spectrometry (MS) and related

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databases, respectively. In recent years, the metabolomic perspectives of numerous fermented

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foods viz., koji, doenjang, and gochujang, have been comprehensively studied to gain insights

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of the corresponding nutritional, functional, and quality biomarkers.10,15,16 Herein, we

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examined the subtle effects of rice substrate milling on metabolomic profiles and associated

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biochemical phenotypes viz., enzyme activities and antioxidant phenotypes.

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2. MATERIALS AND METHODS

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2.1. Chemicals and Reagents. HPLC grade water, methanol, and acetonitrile were obtained

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from Thermo Fisher Scientific (Waltham, MA, USA). Analytical grade chemicals viz., acetic

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acid, 2,2′-azinobis (3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS), casein 4


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from bovine milk powder, Folin-Ciocalteu’s phenol, formaldehyde solution, formic acid,

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methoxyamine hydrochloride, p-nitrophenol, p-nitrophenol β-D-glucopyranoside (p-NPG),

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potassium persulfate, pyridine, sodium hydroxide, sodium acetate, starch, N-methyl-N-

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(trimethylsilyl) trifluoroacetamide, 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid

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(Trolox), and tyrosine were purchased from Sigma-Aldrich (St. Louis, MO, USA). Sodium

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carbonate, sodium dihydrogen phosphate, and disodium hydrogen phosphate were purchased

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from Junsei Chemical Co., Ltd. (Tokyo, Japan). Trichloroacetic acid was obtained from

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Merck Millipore Co. (Darmstadt, Germany).

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2.2. Rice Koji fermentation with substrates having varying DOM and sample harvest.

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Brown rice samples of a Korean cultivar, Jinsang, were used in this study towards the

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fermentative production of rice koji samples and differentially milled using a polishing

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machine (Model MP-220, Yamamoto Co., Tokyo, Japan) to obtain rice samples with different

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DOM. Each DOM was represented in terms of the proportion of embryo bud and bran layer,

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remained in rice (Table 1). The 5 milled rice categories viz., DOM 0, DOM 5, DOM 7, DOM

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9, DOM 11, were collected and stored at -20°C. The koji mold Aspergillus oryzae (KCCM

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12698) procured from the Korean Culture Center of Microorganisms (KCCM, Seoul, Korea),

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was used as culture inoculum. A. oryzae was maintained on Malt Extract Agar (malt extract:

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20 g, glucose: 20 g, peptone: 1 g, and agar: 20 g per liter) at 28°C. The fermentative

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bioprocess steps of koji production were adapted from Lee et al.10 The fermented rice koji

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samples were harvested at every 12 h (up to 96 h) and stored at deep freezing conditions (-

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80°C) until analyses.

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2.3. GC-TOF-MS Analysis. All sample extraction and derivatization steps were performed

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as described by Lee at al.10 Gas chromatography time-of-flight MS (GC-TOF-MS) analysis

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was performed on an Agilent 7890A GC system (Santa Clara, CA, USA) with a Pegasus HT 5



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TOF-MS (Leco Corporation, St. Joseph, MI, USA). An RTx-5MS (30 m length × 0.25 mm

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i.d., J & W Scientific, Folsom, CA, USA) was used a carrier gas (helium) at a constant flow

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rate of 1.5 mL/min. The injector and ion source temperature were maintained at 250°C and

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230°C, respectively. The oven temperature was maintained at 75°C for 2 min, and then

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increased to 300°C at 15°C/min, which was sustained for 3 min. Next, 1 µL of sample was

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injected with a mass scan range of 50–800 m/z. Overall, three samples as well as analytical

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replicates were maintained for each variant.

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2.4. LC-ESI-MS Analysis. The samples were extracted and analyzed for secondary

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metabolites using ultra-high-performance liquid chromatography linear trap quadrupole ion

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trap tandem mass spectrometry (UHPLC-LTQ-IT-MS/MS) and ultra-performance liquid

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chromatography quadruple time-of-flight mass spectrometry (UPLC-Q-TOF-MS), using the

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protocols described by Lee at al.10 Samples were separated on a syncronis C18 column with

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100 mm × 2.1 mm, 1.7 µm particle size (Thermo Scientific). The mass spectra and

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photodiode array range in both positive and negative ion mode were tuned for 100–1000 m/z

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and 200-600 nm, respectively.

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2.5. Data Processing and Multivariate Statistical Analysis. The raw datasets from GC-

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TOF-MS and UHPLC-LTQ-IT-MS/MS analyses were transformed to netCDF (*.cdf) format

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using Leco ChromaTOF and Thermo Xcalibur software, respectively. The respective netCDF

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(*.cdf) files were subjected to MetAlign software (http://www.metalign.nl) mediated data

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processing as described previously by Lee et al.10,

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contained the suitable peak mass (m/z), retention times (min), and peak area information as

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variables, was evaluated using SIMCA-P+ 12.0 software (Umetrics, Umea, Sweden) for

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multivariate statistical (MVS) analysis. The data sets were log-transformed and unit variance

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was scaled prior to principal component analysis (PCA) and partial least-squares discriminant

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The resulting data matrix, which

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analysis (PLS-DA) to compare the five rice koji. Significant differences (p-value < 0.05)

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were tested by one-way analysis of variance using PASW Statistics 18 (SPSS, Inc., Chicago,

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IL, USA). The putative metabolite identification was performed through matching respective

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molecular weights and formula, retention time, mass fragmentation patterns, and UV

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absorbance data available in literature and our in-house library (Table S1 and S2).

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2.6. Enzyme Activities. The enzymatic activity assays including amylase, protease, β-

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glucosidase were conducted according to the protocols described by Lee et al.10 Each rice koji

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sample (10 g) in 90 mL of water was extracted by shaking in an incubator at 120 rpm and

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30°C for 1 h. To evaluate enzymatic activities, filtered supernatants were used.

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2.7. Determination of Antioxidant Activity. To determine the antioxidant activity of rice

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koji samples, ABTS and FRAP assays were performed in triplicate. The ABTS antioxidant

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assay was performed using the method partially adapted from Re et al. and Lee et al.10, 17 For

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analysis, the ABTS stock solution was diluted using distilled water achieving a solution with

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absorbance of 0.7 ± 0.02 at 750 nm. Each sample extract (20 µL) was incubated with ABTS

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solution (180 µL) in 96-well plate, and the reaction was incubated at room temperature for 6

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min under dark, and the absorbance was measured at 750 nm. The FRAP assay was

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performed using a mixture of 300 mM acetate buffer (pH 3.6), 20 mM iron(III) chloride, and

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10 mM TPTZ solution in 40 mM HCl (10:1:1, v:v:v). For analysis, 10 µL of sample was

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added 300 µL of FRAP reagent and incubated at room temperature for 6 min. Absorbance

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was measured at 570 nm. The results are represented as the Trolox equivalent antioxidant

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capacity concentration (mM) per milligram of koji. The standard curves of Trolox ranged

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from 0.0156 to 0.5 mM.

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2.8. Physicochemical Characteristics. For evaluating physicochemical characteristics, 3 g

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of koji samples were extracted with 30 mL of distilled water at 30°C for 1 h and 120 rpm. 7



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The content of total sugar contents at 25°C expressed as °Brix was determined using a digital

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refractometer (HI 96811, Hannah Instruments, Woonsocket, RI, USA). pH was measured

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using a pH meter (Thermo). Titratable acidity was measured by titrating the samples with 0.1

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N NaOH solution to a pH value of 8.4. The amino-type nitrogen contents were estimated by

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adding 36% formaldehyde solution (20 mL) to the titrated samples, followed by sample re-

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titration to pH 8.4 using 0.1 N NaOH solution. Amino-type nitrogen contents were

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quantitatively estimated based on the titrated volume of 0.1 N NaOH and transformed to total

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amino acid content.

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3. RESULTS

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3.1. Metabolic Profiling and Multivariate Analyses for Rice Koji fermented using the

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Substrates with Varying DOMs. Differential metabolomes among the rice koji samples

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made using the substrates with varying DOM were examined using multivariate analyses i.e.,

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PCA and PLS-DA, based on GC-MS and LC-MS datasets. The PCA and PLS-DA model

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based on rice koji samples according to the DOM are shown in Figure 1 and Supplementary

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Figure 1, respectively. The principal component analysis (PCA) is an unsupervised,

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multivariate statistical method for transforming an original set of correlated variables to a

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new set of uncorrelated variables, called principal components (PC). On the other hand,

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partial least squares discriminant analysis (PLS-DA) is commonly applied to evaluate the

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clear distinctions between groups (variables) within the observed datasets.

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The PCA score plot derived from GC-TOF-MS (Figure 1A) and UHPLC-LTQ-ESI-IT-

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MS/MS (Figure B) data sets showed 51.2% (PC1: 39.8% and PC2: 11.4%) and 21.7% (PC1:

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18.6% and PC2: 3.1%) variances, respectively. The PLS-DA model showed a similar pattern

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of metabolic profiles compared to the PCA model indicating a steady alteration in rice koji

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metabolomes during the course of fermentation. Intriguingly, each rice koji with different 8


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DOM substrates exhibited different metabolite changes. However, similar patterns were

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observed between rice koji with substrates having DOM 5 and 7 as well as DOM 9 and 11

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samples.

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Differential variables were selected based on variable importance in projection (VIP >1.0)

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values and p-values (p < 0.05) obtained by PLS-DA model. The variable importance in

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projection (VIP) values reflects the statistical significance of each variable in the model. A

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total of 54 primary metabolites by GC-TOF-MS data (18 amino acids, 7 fatty acids, 11

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organic acids, 15 sugars and sugar alcohols, and 4 others) were identified using standard

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compounds, mass fragment patterns, of NIST and an in-house library (Table S1). Further, 13

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secondary metabolites by UHPLC-ESI-LTQ-MS/MS data were selected as major compounds

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related to the discrimination between rice koji with different DOM (Table S2). In particular,

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apigenin-6-C-glucosyl-8-C-arabinoside, isovitexin-2''-O-glucoside, tricin-O-rutinoside, tricin,

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pinellic acid, and 8 lysophospholipids including lysophosphatidylcholines (lysoPC) and

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lysophosphatidylethanolamine (lysoPE) were identified by comparing the data to an in-house

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library and published literature.

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3.1.1 Temporal Primary Metabolomes for Rice Koji fermented using Substrates with

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Varying DOMs. To visualize discriminative primary metabolites according to fermentation

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time, a heat-map was made based on the GC-TOF-MS data. The trends of variations in

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metabolite levels according to fermentation time are shown in Figure 2, with their relative

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levels are represented as fold-changes normalized to the respective value of unfermented rice

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with different DOM. The trends of sugar and sugar alcohol derivatives viz., adonitol, dulcitol,

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gluconate, glycerate, maltose, myo-inositol, raffinose, sucrose, sorbitol, and xylose, were

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gradually increased. The relative abundance of glucose was increased until 24 h and then

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decreased until 96 h. In amino acid metabolism, the relative levels of serine, glycine, 9



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isoleucine, threonine, valine, methionine, aspartate, alanine, phenylalanine, tryptophan, lysine,

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gaba, ornithine, and pyroglutamate were increased, whereas those of asparagine and

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glutamate were gradually decreased. Among the fatty acids, particularly palmitic acid,

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linoleic acid, elaidic acid, oleic acid, linolenic acid, stearic acid, and myristic acid, were

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increased. Metabolites related to the tricarboxylic acid (TCA) cycle such as citrate, malate,

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fumarate, and succinate increased with fermentation time.

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3.1.2 Relative Disparity in the Levels of Discriminant Metabolites in Rice Koji made

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using the Substrates with Varying DOMs. To illuminate the relative levels of discriminant

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metabolites among rice koji types made using the substrate with varying DOM representing

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each fermentation time point, a heatmap was used for indicating the highest levels of

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respective metabolites (Table 2). The comparison with substrates (unfermented rice) revealed

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that most metabolites such as amino acids, organic acids, sugars and sugar alcohols and

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secondary metabolites were relatively higher in DOM 0 koji, whereas fatty acids were highest

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in DOM 5 koji. Depending upon the fermentation time point, the levels of metabolites in

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various rice koji types changed from DOM 0 to DOM 5–7 at 24 h, and subsequently from

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DOM 5–7 to DOM 7–9 at 36 h. Between 48–72 h, these metabolic trends changed in the rice

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koji made with substrates having DOM 7–11. In contrast, the highest levels of amino acids,

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sugars and sugar alcohols, nucleotide, fatty acids, and organic acids except secondary

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metabolites were observed in DOM 0 rice koji samples at 96 h.

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3.2 Variations in Enzymatic Production, Bioactivities, and Physiological Characteristics

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for Rice Koji Types made using the Substrates with Varying DOM. The fermentation time

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correlated enzymatic activities for different rice koji types made using the substrates with

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varying DOM were determined to estimate the differential secretion of hydrolytic enzymes

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by A. oryzae (Figure 3 A–C). In general, the amylase, protease, and β-glucosidase activities 10


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were increased linearly with fermentation time in different rice koji types. The protease and

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β-glucosidase activities were highest in the koji samples with DOM 7 substrates, whereas

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amylase activity showed higher values in koji samples with DOM 0–9 substrates, except for

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samples with DOM 11. Interestingly, amylase activity for all koji types regardless of DOM

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was highest at 24 h, while protease and β-glucosidase activities increased linearly until 96 h.

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Measurement of the functional phenotypes, i.e., ABTS and FRAP, to determine antioxidant

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activities showed that the rice koji samples with DOM 0 substrates had the highest

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antioxidants levels followed by DOM 5 > DOM 7 >DOM 9, 11 substrates made rice koji

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(Figure 3 D and E). Hence, we assume that the antioxidant activities of koji increased linearly

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until 96 h, regardless of DOM of the substrates.

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4. DISCUSSION

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We employed the metabolomic approaches to evaluate the comprehensive metabolic and

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biochemical events underlying fermentative koji preparation with rice substrates having

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varying DOM. We observed that the differential metabolomes of rice koji types were the

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direct biochemical functions of (1) time-correlated metabolism, and (2) enzyme activities, by

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koji mold subjected to fermentation on rice substrates with varying DOM. During the course

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of fermentative growth, the spore count for A. oryzae was steadily increased while the

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moisture content was decreased. There were no significant differences observed for mold

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growth and sporulation among the koji samples with 5 different DOMs. We assumed that the

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decrease of moisture content in koji was probably due to consumption of water by Aspergillus

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growth as well as through evaporation.18

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The multivariate analysis for various rice koji extracts based on GC-TOF-MS datasets

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indicated a fermentation time-correlated metabolic profile for primary metabolites regardless

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of the DOM (Figure 1A). On the other hand, the multivariate analysis based on UHPLC11



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LTQ-ESI-IT-MS/MS datasets showed a secondary metabolite profile depending on various

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DOM of rice substrates (Figure 1B). As shown in Figure 2, the primary metabolites altered

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briskly with enzyme activities over time i.e., increased abundance of metabolites related to

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sugar metabolism with increase in amylase and β-glucosidase activities at 24 h and 72 h,

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respectively. Similarly, the gradual changes in amino acid levels were recorded with

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enhanced protease activity. The fatty acid levels were also increased at 24 h.

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4.1. Time Correlated Metabolic Alterations for Rice Koji Independent of Substrate's

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DOM. The organic acids produced in the TCA cycle viz., citrate, fumarate, malate, and

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succinate acts as intermediates to various metabolic pathways (Figure 2). With an increase in

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organic acids contents, the pH decreased with an accompanying increase in titratable acidity

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of the rice koji samples (Figure 4C and D). The production of citric acid used in the dairy,

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food, beverage, pharmaceutical, and biochemical industries is extensively carried out using

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Aspergillus species.19 The fumaric acid is used as a starting metabolite for the synthesis of

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polymers, while the malate and succinate has wide applications in the food, beverage, and

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pharmaceuticals industries.20

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Among the sugars, the glucose levels showed the highest levels at 24 h unlike other sugar

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metabolites, followed by its steep decline levels during the later stages of koji fermentation.

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In particular, glucose is a major carbon source hydrolyzed from the substrate starch by fungal

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enzymes including amylase and β-glucosidases. The sugars are further metabolized to sugar

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alcohols through alcoholic fermentation.21 The sugar alcohols are produced during microbial

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fermentation through multiple fermentation pathways.22 For example, erythritol, a four-

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carbon sugar alcohol that is widely distributed in nature such as in foods, is produced

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industrially via the pentose phosphate pathway beginning with enzymatic hydrolysis of the

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starch from rice to generate glucose. Similarly, Sorbitol can be obtained by simultaneous 12


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hydrolysis and reduction of glucose to change the aldehyde group to a hydroxyl groups.

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The proteolytic enzymes produced by Aspergillus species release the organic nitrogen

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contents from complex proteins to be used as a nitrogen source essential for growth and

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metabolism.23,24 The rice-koji fermentation with Aspergillus spp. effectively increased the

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contents of free amino acids (alanine, glycine, and serine) related to sweetness and nutritional

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properties.25 The total amino acid contents including those of aromatic amino acids

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(phenylalanine and tryptophan) and branched-chain amino acids (leucine, isoleucine, and

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valine) were increased with enhanced protease activities over time (Figure 2, 3B, & 4F).

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Functionally, the biosynthesis of aromatic amino acid in A. fumigatus has been linked with its

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antifungal properties, while the branched-chain amino acids in A. nidulans are reportedly

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vital for building proteins.26,27 In our study, we observed a decrease in glutamate contents

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coupled with an increase in other amino acids contents, especially those of γ–aminobutyric

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acid (GABA) over time. It has earlier been reported that, A. oryzae contains a gene for

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glutamate decarboxylase (GAD, E.C. 4.1.1.15) which produces GABA via decarboxylation of

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glutamic acid during cultivation of the spore suspension.28 GABA was employed in

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functional foods for reducing blood pressure, promoting better sleep, and diuretic effects.29-31

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The relative levels of unsaturated fatty acids, saturated fatty acids, and lysophospholipids

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were increased with the biosynthesis as well as bioconversion of fatty acids (Figure 2).

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Whole rice bran lipids induce the lipid metabolism. This means that fungi generate excess

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lipids by fermenting the substrate materials as well as synthesize their own lipids for fungal

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biomass production.32 Earlier, Abu et al. have reported that solid-state fermentation with A.

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oryzae typically increases the concentrations of lipids, primarily including C16:0, C18:0, and

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C18:1.33

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4.2. Metabolic Alterations for Rice Koji Depending on Substrate's DOM. A rice grain has 13



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different biomolecular compositions for its different parts arranged from surface to inside

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including outer husk or hull, bran, embryo, and innermost endosperm. A variety of

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metabolites in rice viz., phenolic acids, cinnamic acids, anthocyanins, flavonoids, steroidal

283

compounds, polymeric carbohydrate, and proteins are nutritionally vital as health-promoting

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functional foods.34 Following the milling process, rice contains less of the embryo bud and

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bran layer (Table 1). We detected higher proportions of amino acids, organic acids, sugars and

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sugar alcohols, and lysophospholipids together contributing to the high antioxidant activities

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for DOM 0 rice koji as compared to those for DOM 5–11 koji samples (Table 2, Figure 3 D

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and E), maintaining similar pH and titratable acidity (Figure 4 C–D). Though, the relative

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abundance for most metabolites in rice koji, made using the substrates with intermediate

290

DOM, were increased during the course of fermentation, the metabolite abundance for

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unfermented DOM 0 substrate was comparatively higher than substrates with increased DOM

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(Table 2).

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Given the proportion of the bran layer in rice after different DOM, rice koji was relatively

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affected by various microbial enzymes and associated bioactivities. The protease and β-

295

glucosidase activities were observed varying in the order of substrate's DOM 5 > 7 > 0 > 9 >

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11, while the amylase activity was observed unaffected by the DOM. Accordingly, amino

297

acids, sugars, and sugar metabolism were highest in DOM 5-7 sample during the middle of

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fermentation at 24–72 h. Intriguingly, most of the primary metabolites were highest in the

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koji samples with DOM 0, at the end 96 h fermentation. Mechanistically, β-Glucosidase

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cleaves glucosidic linkages in polysaccharides and flavonoid glycosides and hydrolyzes them

301

into oligo- or mono-saccharides and corresponding flavonoid aglycones to improve

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enzymatic saccharification.35 In the koji samples with DOM 5 substrate, enhanced β-

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glucosidase activities in rice koji were mainly correlated with a decrease in the flavonoid 14


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glycosides and tricin rutinoside with corresponding increase in the relative levels of sugars

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and flavonoid aglycone, tricin. The changes observed in the metabolite levels for DOM 0 koji

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samples might have been impeded by the rice bran wall impervious to enzymatic hydrolysis.

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Hence, following the rice bran hydrolysis, the relative levels of most metabolites was altered

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in DOM 0 koji samples. The delay in metabolite release in case of brown rice koji

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fermentation was reportedly linked to the impenetrable brine layer, which limits enzymatic

310

penetration and subsequent hydrolysis.10 Earlier, Yu et al. have reported that the protease and

311

glucoamylase activities for rice koji made using a brown rice substrate with 50–70% DOM

312

reach maximum following the 5 days fermentation with A. oryzae.36

313

The ABTS and FRAP assay determined antioxidant activities were considerably higher

314

in rice koji using substrates with DOM 0, 5, and 7, in comparison to those for DOM 9 and 11

315

koji samples, during the courses of fermentation. Hence, increasing the substrate's DOM

316

allows the enzymatic penetration through impervious rice bran and endosperm, hence

317

releasing the polyphenols, flavonoids, and free amino acids affecting the antioxidant

318

activities.13,14 Antioxidant activity prevents or limits oxidation, which is considered beneficial

319

for improving food quality and health. Evaluating antioxidant compounds from different

320

DOM of rice substrates provides useful information for commercial koji making owing to the

321

nutritional and functional properties of antioxidant metabolites.

322

Previously, it has been shown that flavonoid and phenol compounds derived from plants

323

possess high antioxidant activities.37 Pérez et al. have reported that the amino acid

324

composition of honey affects its free radical scavenging capacity.38 A known antioxidant

325

phytochemical i.e., ferulic acid is commonly found in rice bran, which reportedly alleviates

326

oxidative stress in various organic systems.39 We observed that the 96 h Rice koji with DOM

327

0 substrate contained the highest levels of antioxidant compounds. We conjecture that 15



328

antioxidant potentials of rice koji are mainly influenced by its flavonoids and phenolic acid

329

components, which synergistically constitutes to its various functional properties.

330

In conclusion, we observed that A. oryzae exhibited a unique overall metabolism

331

maneuvering the secretion of hydrolytic enzymes (amylase, β-glucosidase, and proteases)

332

ergo the metabolomes in rice koji fermented using the substrates with varying DOM.

333

Typically at the initial and final stages of koji fermentation, a variety of functional

334

metabolites were relatively higher in koji made with substrates having DOM 0, in comparison

335

to koji types made with substrates subjected to varying DOM. However, in the middle stages

336

of koji fermentation, the substrates with DOM 5–7 showed relatively higher contents of

337

metabolites intermediates of carbohydrate metabolism viz., sugars and sugar alcohols, organic

338

acids, phenolic acids, as well as lipid metabolism intermediates such as fatty acids and

339

lysophospholipids. The enhanced release of free metabolites in the koji samples was

340

influenced by the relatively higher amylase, β-glucosidase, and protease levels in rice

341

substrates with DOM 5-7 as compared to DOM 0. The present findings provide useful

342

insights for large-scale commercial production of rice koji that relies heavily on the DOM to

343

rationalize its raw substrate processing.

16


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344

AUTHOR INFORMATION

345

Corresponding Author

346

*Phone: +82-2-2049-6177; fax: +82-2-455-4291; E-mail: [email protected]

347

Notes

348

The authors declare no competing financial interest.

349 350

SUPPORTING INFORMATION

351

The list of significantly distinct metabolites from rice koji with different DOM during

352

fermentation identified by GC-TOF-MS (Table S1), list of significantly distinct metabolites

353

from rice koji with different DOM during fermentation identified by UHPLC-LTQ-ESI-IT-

354

MS/MS (Table S2), partial least square discriminant analysis (PLS-DA) score plot derived

355

from the GC-TOF-MS (A) and UHPLC-LTQ-ESI-IT-MS/MS data set of rice koji cultured

356

with A. oryzae according to the degree of milling (DOM) (Figure S1).

357 358

REFERENCES

359

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CRC Press, (Chapter 1), 2014.

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after solid-state fungal fermentation. Bioresour. Technol. 2011, 102, 8335-8338.

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protein composition of sweet potato (Ipomoea batatas) after solid-state fungal fermentation.

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34. Friedman, M. Rice brans, rice bran oils, and rice hulls: composition, food and industrial

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uses, and bioactivities in humans, animals, and cells. J. Agric. Food Chem. 2013, 61, 10626-

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10641.

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determining efficiency of cellulose and biomass hydrolysis. J. Bioprocess Biotech. 2014, 5, 1-

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8.

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36. Yu, K.W.; Lee, S.E.; Choi, H.S.; Suh, H.J.; Ra, K.S.; Choi, J.W.; Hwang, J.H.

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Optimization for rice koji preparation using Aspergillus oryzae CJCM-4 isolated from a

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korean traditional meju. Food Sci. Biotechnol. 2012, 21, 129-135.

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37. Kong, S.; Lee, J. Antioxidants in milling fractions of black rice cultivars. Food Chem.

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2010, 120, 278-281

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38. Pérez, R.A.; Iglesias, M.T.; Pueyo, E.; González, M.; de Lorenzo, C. Amino acid

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360-365.

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39. Srinivasan, M.; Sudheer, A.R.; Menon, V.P. Ferulic acid: Therapeutic potential through its

469

antioxidant property. J. Clin. Biochem. Nutr. 2017, 40, 92-100.

470 471

FUNDING SOURCES

472

This work was supported by the National Research Foundation of Korea(NRF) grant funded

473

by the Korea government(MSIP) (No. NRF-2017M3C1B5019303) and funded by the

474

Strategic Initiative for Microbiomes in Agriculture and Food, Ministry of Agriculture, Food

475

and Rural Affairs, Republic of Korea (as part of the (multi-ministerial) Genome Technology 22


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476


to Business Translation Program) (grant number 916005-2).

23



477

FIGURE CAPTIONS

478

Figure 1. Principal component analysis (PCA) score plot from the GC-TOF-MS (A) and

479

UHPLC-LTQ-IT-MS/MS (B) data set of rice koji cultured with A. oryzae according to the

480

degree of milling (DOM) ) (◆, ◆, 0; ◆, 5; ◆, 7; ◆, 9; ◆, 11 ; +, 0 hr; ✱, 12 h; ▲, 24 h; ■,

481

36 h; ▼, 48 h; ●, 72 h; ◆, 96 h).

482 483

Figure 2. Pathway for the relative levels of discriminant metabolites at different times of rice

484

koji with DOM as determined using the PLS-DA datasets (variable importance in projection

485

> 1.0, p-value < 0.1) for GC-TOF-MS analyses. The discriminant metabolites were further

486

correlated with corresponding steps in the biosynthetic pathways adapted from KEGG

487

database. The values represent the fold-change with respect to unfermented rice.

488 489

Figure 3. Changes in amylase (A), protease (B), β-glucosidase (C), ABTS (D), and FRAP (E)

490

of rice koji cultured with A. oryzae according to DOM during fermentation (■, 0; ■, 5; ■, 7;

491

■, 9; ■, 11).

492 493

Figure 4. Total mold count (A), moisture contents (B), pH (C), titratable acidity (D), total

494

sugar content (E), and total amino acid content (F) of koji cultured with A. oryzae according

495

to the degree of milling (DOM) during fermentation (■, 0; ■, 5; ■, 7; ■, 9; ■, 11).

24


Page 24 of 33

Page 25 of 33


496 497

Figure 1.

498

25



499 500

Figure 2.

501

26


Page 26 of 33

Page 27 of 33


502 503

Figure 3.

504

27



505 506

Figure 4.

507

28


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Page 29 of 33

508


Table 1. Information for degree of milling of rice .

Degree of milling

Proportion of the embryo bud and

(DOM)

bran layer in rice (%)

0

100

5

50

7

30

9

15

11

5

29


509


Page 30 of 33

510

Table 2. Column representations for the highest levels of discriminant metabolites in 5 rice

511

koji according to the DOM. Each column represents DOM color having metabolite of the

512

highest content at each time. (■, 0; ■, 5; ■, 7; ■, 9; ■, 11)

Fermentation time Metabolites

0h 7

12 h

0

5

9

11

0

alanine

'

'

'

'

'

asparagine

'

'

'

'

aspartic acid

'

'

'

γ-Aminobutyric acid

'

'

glutamic acid

'

glycine

24 h

5

7

9

11

0

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Isoleucine

'

'

'

'

leucine

'

'

'

lysine

'

'

methionine

'

orinithine

36 h

5

7

9

11

0

'

'

'

'

'

'

'

'

'

'

'

'

'

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'

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'

'

'

'

'

'

'

'

'

'

'

phenylalanine

'

'

'

'

'

'

proline

'

'

'

'

'

pyroglutamic acid

'

'

'

'

serine

'

'

'

threonine

'

'

tryptophan

'

valine

48 h

5

7

9

11

0

'

'

'

'

'

'

'

'

'

'

'

'

'

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'

'

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'

'

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'

'

'

myristic acid

'

'

elaidic acid

'

linoleic acid

72 h

5

7

9

11

0

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

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'

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'

'

'

'

linolenic acid

'

'

'

'

oleic acid

'

'

'

palmitic acid

'

'

stearic acid

'

'

96 h

5

7

9

11

0

'

'

'

'

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Amino acids

Fatty acids

Organic acids lactic acid

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Page 31 of 33


4-hydroxybenzoic acid

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acetic acid

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citric acid

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ethanolamine

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fumaric acid

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malic acid

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salicylic acid

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shikimic acid

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succinic acid

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ferulic acid

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glycerol

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adonitol

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dulcitol

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gluconic acid

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Sugar & sugar alcohols

glucose glyceric acid

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glyceryl-glycoside

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maltose Erythritol

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myo-inositol

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N-acetyl-D-glucosamine

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raffinose

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sorbitol

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sucrose

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xylose

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adenine

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guanine

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olemide

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uridine

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Nucleosides

Secondary metabolites apigenin-6-C-glucosyl-8-C-arabinoside isovitexin-2''-O-glucoside tricin-7-O-rutinoside tricin pinellic acid lysoPE14:0

31



lysoPC18:3 lysoPC14:0 lysoPE18:2 lysoPC16:1 lysoPC18:2 lysoPE16:0 lysoPC18:1

513 514

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Page 33 of 33

515


GRAPHIC FOR TABLE OF CONTENTS

516 517 518 519

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Metabolomics reveal optimal grain pre-processing (milling) toward rice

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